Some internal combustion engines include engine control systems that deactivate cylinders under low load situations. For example, an eight cylinder engine can be operated using four cylinders to improve fuel economy by reducing pumping losses. This process is generally referred to as displacement on demand or DOD. Operation using all of the engine cylinders is referred to as an activated mode. A deactivated mode refers to operation using less than all of the cylinders of the engine (one or more cylinders not active).
In the deactivated mode, there are fewer cylinders operating. As a result, there is less drive torque available to drive the vehicle driveline and accessories (e.g., alternator, coolant pump, A/C compressor). Engine efficiency, however, is increased as a result of decreased fuel consumption (i.e., no fuel supplied to the deactivated cylinders) and decreased engine pumping. Because the deactivated cylinders do not take in and compress fresh intake air, pumping losses are reduced.
During typical engine operation in the deactivated mode, there are many operating points at low intake manifold absolute pressures (MAP). At low MAPs, the engine pumping losses are higher than at higher MAPs. That is to say, the engine is still not operating as efficiently as desired when operating in the deactivated mode at a low MAP. Additionally, there are many operating points at or near the MAP threshold, which defines the transition point between activated and deactivated modes. Once in the deactivated mode, hysteresis often delays transition back into the deactivated mode. Both of these conditions result in missed opportunities to reduce fuel consumption.